Rainfall this month has already surpassed what is normal for May in Baltimore, with several more inches expected to bring flooding risks through the weekend.

So, why is the precipitation so persistent?

It’s the result of an atypical weather pattern sending Gulf moisture north and dumping it onto the Mid-Atlantic.

“It’s unusual to have so many straight days of rainfall,” said James Lee, the meteorologist in charge of the National Weather Service’s Baltimore/Washington forecast office.

The first piece of the pattern is a so-called Bermuda high, a pressure system with clockwise circulation that commonly begins to appear this time of year. Centered over or near Bermuda, it pumps hot, moist air from the south and southwest into Maryland, creating many memorable summer heat waves here.

Photos of weather in Maryland in 2018.

But two other weather systems are interacting with the Bermuda high in a way that is dousing the Mid-Atlantic.

One is an area of low pressure over the Southeastern U.S. Low pressure systems circulate counter-clockwise, so that system and the Bermuda high system are acting like two tops spinning in opposite directions, funneling the Gulf moisture in one direction — right up the Interstate 95 corridor, Lee said.

Then, there is a high-pressure system over Eastern Canada blocking the moisture from coming farther north. A frontal boundary at the southern edge of that area has been stubbornly stationary right over Maryland, wringing out the tropical moisture when it reaches the region.

Since Sunday, the pattern has produced more than 3 ½ inches of rainfall at Baltimore-Washington International Thurgood Marshall Airport, and even more in other parts of the region.

Forecasters expect 3-5 more inches of rain over the next few days, with a flood watch in effect from Thursday afternoon through Saturday morning. As soil becomes increasingly saturated, it will take less rain to cause floods.

“This rain will be heavy at times,” forecasters wrote. “While flash flooding cannot be ruled out, the primary concern is flooding of small streams and low-lying areas. Streams are already elevated and soils saturated from earlier rainfall, increasing the flood threat.”

Normal May rainfall in Baltimore is 3-4 inches, and there was 3.6 inches of rainfall at BWI, the region’s point of record, from May 1 through Wednesday.

That means this month could be Baltimore’s wettest May since 2009, when 8.42 inches fell. That was the region’s second-wettest May on record; the wettest came in 1989, with 8.71 inches of rain.

Meteorologists expect the wet weather pattern to break down Sunday, but only briefly.

“Then we’re going to go back into another similar-like pattern early next week,” Lee said. “Maybe not quite as moist. But really, the next 10 days is looking like a pretty moist pattern for the mid-Atlantic.”

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Barbara McLewee of Churchville talks about the red oak tree that fell and hit her house (Lloyd Fox/Baltimore Sun video)

Barbara McLewee of Churchville talks about the red oak tree that fell and hit her house (Lloyd Fox/Baltimore Sun video)

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Baltimore County Fire spokesman Tim Rostkowski talks Friday afternoon about the death of a Kingsville woman, likely as the result of a fallen tree limb. (Lloyd Fox/Baltimore Sun video)

Baltimore County Fire spokesman Tim Rostkowski talks Friday afternoon about the death of a Kingsville woman, likely as the result of a fallen tree limb. (Lloyd Fox/Baltimore Sun video)